Immunoglobulin diversity. 1) Variable region amino acid sequences have been determined for 3 IgM, Kappa Beta(1,6) galactan binding monoclonal antibodies derived from the same fusion. These sequences in conjunction with Southern blot analysis indicate that the 3 antibodies derive from a common precursor even though amino acid substitutions are found in the variable regions. These results suggest that the amino acid substitutions result from somatic point mutations which occur in a continuous manner during ontogeny and are not associated with immunoglobulin class switching. To further establish the nature of the observed amino acid substitutions, the entire gene family encoding these heavy chains has been cloned and sequenced. None of the variant protein sequences were found to be encoded in germ line genes confirming their somatic origin. 2) The question of multigene evolution and mutation is being approached by an analysis of immunoglobulin genes isolated from a variety of mouse species and sub-species representing a spectrum of the evolution of this genus. Genomic libraries have been constructed from four different species and appropriate immunoglobulin genes are being isolated for DNA sequence analysis.